1. Field of the Invention
The present invention generally relates to fluid supply apparatuses and image forming apparatuses, and more particularly to a reversible fluid supply apparatus in which the direction of transport of a fluid can be switched, and to an image forming apparatus equipped with such a fluid supply apparatus.
2. Description of the Related Art
There are various types of image forming apparatus, such as printers, facsimile machines, copiers, plotters, and multifunction peripherals. Inkjet recording apparatuses are an example of the image forming apparatus of a fluid-discharge recording type. In this type of image forming apparatus, typically droplets of ink are discharged out of a recording head onto a recording medium as the recording medium is transported, thereby forming an image on the recording medium. By “forming an image” is meant that an image is either recorded, transferred, or printed, for example.
There are two types of inkjet recording apparatus. One is the serial-type by which droplets of ink are discharged while the recording head is moved in a main scan direction, thereby forming an image on the recording medium. The other is the line type which employs a line head to discharge ink droplets to form an image without moving the recording head.
The term “recording medium” includes a sheet of paper, threads, fibers, fabrics, leather, metals, plastics, glass, wood, and ceramics, for example. The term “image” formed on the recording medium includes both images with some meaning, such as a certain character or a figure, and images without particular meanings, such as a random pattern, which may be formed by landing ink droplets on the medium randomly.
The term “ink” is intended to refer to any fluid with the use of which an image can be formed. Such fluid may be referred to as a recording fluid or a fixing solution. The term “sheet” is intended to refer to not just a sheet of paper but also any sheet on which ink droplets can attach, such as an overhead projector (OHP) sheet, cloth, etc. The “sheet” therefore include a recording medium, a recording paper, a recording sheet, and the like.
In the fluid discharge type of image forming apparatus, two ways of supplying ink to the recording head are known. In one, an ink cartridge, i.e., an ink container, is removably attached to the carriage. The ink is supplied from the ink cartridge to the recording head, which is mounted on the carriage. In the other, a smaller-volume sub-tank (also referred to as a “head tank” or a “buffer tank”) is mounted on the carriage. The sub-tank supplies ink to the recording head. A larger-volume ink cartridge (main tank) is replaceably attached to the apparatus main body. The sub-tank is refilled with ink from the main tank on the main apparatus side.
In the latter system, in which the main tank can be replaced, the main tank and the head tank are connected via an ink supply channel (fluid supply channel). The fluid supply channel is often made from a flexible resin tube for ease of handling within the apparatus, assembly, and maintenance.
As the speed of printing operations increases, there is a corresponding demand for faster carriage movement. This leads to increased vibrations in the fluid supply channel, such as a supply tube, as it follows the moving carriage. As a result, pressure variations are caused in the ink in the supply channel relative to the recording head. In order to reduce the pressure variations, various complex pressure damping mechanisms have been devised. A structure has also been adopted in which fluid communication between the main tank and the sub-tank is disconnected by blocking the supply channel with a valve or the like.
Because the soft resin tubes have a low gas barrier property, external air may enter (or permeate) the tube when used as the fluid supply channel, producing bubbles in the tube. Air may also enter the supply channel via the connecting portion between the supply channel and the main tank or the sub-tank. If air enters the supply channel, the air reaches the sub-tank due to a predetermined negative pressure formed therein, thereby reducing the pressure within the sub-tank and causing the ink to drip out of the nozzle surface of the recording head.
To overcome this problem, the air that entered the sub-tank may be discharged to the atmosphere, followed by sucking the ink out of the nozzle surface of the recording head using a suction pump, so that a predetermined pressure (negative pressure) can be restored in the sub-tank. In this method, however, a large amount of waste ink is produced by the sucking of ink from the nozzle opening, which is not economical.
In another solution, the ink within the supply channel may be fed back to the main tank in order to control the pressure within the head tank, using various methods or pumps for changing fluid transport directions for suction or discharge.
For example, Japanese Laid-Open Patent Application No. 2004-351641 (hereafter referred to as “Patent Document 1”) discloses a system in which parallel circulation channels are formed such that they do not pass near the recording head nozzle, wherein the ink and bubbles within the sub-tank are fed back to the main tank by a force-feed unit.
Japanese Laid-Open Patent Application No. 2002-370374 (hereafter referred to as “Patent Document 2”) discloses a system in which the internal volume of the sub-tank is compulsorily changed in a decreasing direction so that the bubbles within the sub-tank can flow back to the main tank together with the ink.
Japanese Laid-Open Patent Application No. 60-256577 (hereafter referred to as “Patent Document 3”) discloses a pump system in which a shell is rotatably supported within a casing having a suction opening and a discharge opening. The inside of the shell is partitioned by a diaphragm into a pump chamber and an operating fluid chamber. The pump chamber has an opening that can be communicated with the suction opening or the discharge opening as the shell is rotated.
However, in the system according to Patent Document 1, the details of the ink force-feed unit are unclear, and the system requires the two parallel circulating channels for each color. As a result, the arrangement of the channels within the apparatus is complicated, making assembly of the apparatus difficult and requiring an increased cost. The system according to Patent Document 2 requires the pressurizing/depressurizing unit for pressurizing or depressurizing the sub-tank in order to increase or decrease the volume of the sub-tank. Furthermore, a valve unit needs to be installed outside the carriage. As a result, the apparatus increases in size and complexity. In the pump according to Patent Document 3, the diaphragm cannot ensure sufficient resistance against the permeability of the ink.
As a general bidirectional pump capable of sucking and discharging fluid, tubing pumps are known. The tubing pump, however, is problematic in barrier property against gas permeation and durability due to its structure in which a low-hardness resilient tube is compressed by pressing rollers.